Technical Field
The present invention generally relates to the control of threshold voltage for a semiconductor device and more particularly, the present invention relates to a method of making and gate structure for controlling threshold voltage in a midgap semiconductor device.
Background Information
Where multiple different types of semiconductor devices, such as transistors (e.g., MOSFETs (Metal-Oxide Semiconductor Field-Effect Transistor)), memory and other types are used in the same application, different threshold (i.e., turn-on) voltages (Vth) are needed. For example, SRAM (Static Random Access Memory) typically requires a relatively higher Vth to reduce leakage. As another example, n-type and p-type logic elements typically have relatively low threshold voltages, yet still distinct from each other. Other applications may benefit from a Vth that lies somewhere in between that of the noted n and p-type elements; also referred to as “midgap” devices or elements.
However, the traditional techniques to manipulate Vth have proved troublesome for various reasons. For example, channel doping has been used for transistors to achieve a desired Vth. While this approach works satisfactorily with planar transistors, it is difficult to control the doping level in three-dimensional structures, such as FinFETs, resulting in Vth non-uniformity. Another approach has been to control the thickness of work function or barrier layers. However, threshold voltage control by work function layer thickness or barrier thickness alone is very sensitive to relatively small thickness changes. For example, in p-type field effect transistors (PFETs), the threshold voltage can be controlled by the thickness of the work function layer, such as titanium nitride (TiN).
However, a change of even a few Angstroms of thickness can cause a Vth shift of more than 100 mV. Similarly, in a n-type field effect transistor (NFET), a threshold voltage can be controlled via the thickness of the barrier layer, such as titanium carbide (TiC), tantalum carbide (TaC) or titanium aluminum (TiAl). Similarly, a thickness change of a few Angstroms results in a comparable Vth shift, and it is difficult to precisely control the barrier layer thickness. Thus, a more stable and less sensitive method is needed.